[ELF] - Fail when multiple .debug_* sections are used in a single object.

[grimar]

Summary

It is based on discussions in "[llvm-dev] [RFC] - Deduplication of debug information in linkers (LLD)." thread.
DWARF5 specification (http://dwarfstd.org/doc/DWARF5.pdf) mentions it can be one or more of debug sections.
For example:

1. p366 says objects may have multiple .debug_abrev, .debug_info, .debug_line sections for DWARF elimination.
2. p376 says multiple .debug_info section could be used for deduplication of type units.

It is unclear that we will want to support the optimizations above and in both cases LLD would work incorrectly now
(as we do not expect multiple of above sections) for cases when we are trying to work with DWARF sections.
It is error reporting case and --gdb-index generation case.
So for both safety and simplicity of implementation I suggest to error out when LLDDwarfObj meets object with
any multiple debug sections with the same name.

[grimar]

This would not work for case when we have multiple .debug_type sections,
though it is legal and already supported. I'll update patch with the testcase for that
(if approach chosen by this patch be approved during current discussion in mail list).

[grimar]

It is too old.

ELF/GdbIndex.h

template <class ELFT> class LLDDwarfObj final : public llvm::DWARFObject {
   StringRef GnuPubNamesSection;
   StringRef GnuPubTypesSection;
   StringRef StrSection;
 
   template <class RelTy>
   llvm::Optional<llvm::RelocAddrEntry> findAux(const InputSectionBase &Sec,
                                                uint64_t Pos,
                                                ArrayRef<RelTy> Rels) const;
+  // Used to detect multiple debug sections with the same name.
+  llvm::StringSet<> Seen;
 
 public:
   explicit LLDDwarfObj(ObjFile<ELFT> *Obj);
   const llvm::DWARFSection &getInfoSection() const override {
     return InfoSection;
   }
   const llvm::DWARFSection &getRangeSection() const override {
     return RangeSection;

ELF/GdbIndex.cpp

 
 using namespace llvm;
 using namespace llvm::object;
 using namespace lld;
 using namespace lld::elf;
 
 template <class ELFT> LLDDwarfObj<ELFT>::LLDDwarfObj(ObjFile<ELFT> *Obj) {
   for (InputSectionBase *Sec : Obj->getSections()) {
-    if (!Sec)
+    if (!Sec || !Sec->Name.startswith(".debug"))
       continue;
+
+    if (!Seen.insert(Sec->Name).second)
+      error(toString(Obj) + ": multiple " + Sec->Name +
+            " sections are not supported");
+
     if (LLDDWARFSection *M = StringSwitch<LLDDWARFSection *>(Sec->Name)
                                  .Case(".debug_info", &InfoSection)
                                  .Case(".debug_ranges", &RangeSection)
                                  .Case(".debug_line", &LineSection)
                                  .Default(nullptr)) {
       Sec->maybeUncompress();
       M->Data = toStringRef(Sec->Data);
       M->Sec = Sec;

test/ELF/multiple-debug-sections.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t1.o
+# RUN: not ld.lld --gdb-index %t1.o -o %t 2>&1 | FileCheck %s
+# CHECK: error: {{.*}}.o: multiple .debug_info sections are not supported
+# CHECK: error: {{.*}}.o: multiple .debug_abbrev sections are not supported
+# CHECK: error: {{.*}}.o: multiple .debug_line sections are not supported
+
+.section  .debug_info,"",@progbits
+.quad 0
+
+.section  .debug_info,"G",@progbits,foo,comdat
+.quad 0
+
+.section  .debug_abbrev,"",@progbits
+.quad 0
+
+.section  .debug_abbrev,"G",@progbits,foo,comdat
+.quad 0
+
+.section  .debug_line,"",@progbits
+.quad 0
+
+.section  .debug_line,"G",@progbits,foo,comdat
+.quad 0